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Ordinary moment-resisting frames are studied for multiobjective design under seismic loading, using the ant colony optimisation method. The objective function contains minimisation of the total weight as well as the corresponding seismic base shear. The modal spectral method is applied for the analysis of ordinary moment-resisting frames. The structure is under strength and serviceability constraints, enforced by penalty functions. All design requirements correspond to uniform building code (UBC) and American Institute of Steel Construction allowable stress design codes of practice. The frame sections are selected from the standard set of American Institute of Steel Construction W-shapes. A new type of initial pheromone depositing is introduced in the ant colony optimisation. Also, to enable improvements on the results, a novel local search is developed and applied to moment frames. For the purpose of comparison, a genetic algorithm is applied to the same problems.